In the past few decades, cognitive neuroscience has experienced an explosion in research investigating the functions of human prefrontal cortex (PFC). One hypothesis that we have championed is that a function of PFC, perhaps specific to the posterior part of the left, inferior frontal gyrus (LIFG), is the selection of information from competing alternatives. In the current proposal, we aim to explore the relation between this putative selection mechanism and the language deficits that have often been associated with brain damage to this region. For more than a century, it has been known that lesions to the left frontal lobe produce linguistic deficits that are characteristically described as "nonfluent," ranging from no language output to truncated, agrammatical phrases. In recent years, however, it has been possible to move beyond broad anatomical correlations with a clinical description of aphasia syndromes, based in large part on the advent of structural and functional neuroimaging techniques. The experiments in this proposal are designed to take advantage of both of these techniques, in order to better describe structure-function relations in PFC, and to understand the link between these functions and the clinical picture of nonfluent aphasia that is associated with damage to prefrontal regions. The major aims of this line of research are as follows: (1) to further develop a unified theory of the functions of PFC; (2) to relate this theory to the clinical syndromes associated with damage to this region of cortex; (3) to clarify inconsistencies regarding symptomatology and lesion localization of nonfluent aphasias with our theoretical approach. The proposal is divided into four sections. In Section I (Verbal Fluency) we will examine the effects of competition and set size on semantic and phonemic fluency. In Section II (Picture Naming) we will examine competition and context effects on lexical retrieval. In Section III (Lexical Ambiguity) we will examine the inhibition of multiple meanings during the resolution of ambiguous words. In Section IV (Syntactic Processing) we will examine the effects of syntactic complexity and ambiguity on sentence comprehension.